1. Field of the Invention
This invention relates to light emitting devices comprising an electroluminescent element such as an LED or an organic light emitting diode (OLED).
2. Description of the Related Art
Light Emitting Diodes (LEDs) have been known for a long time. They are available in many forms. Usually they comprise an LED chip—being an example of an electroluminescent element—with appropriate current supply means and a housing. They are a mass product and available for a large variety of wavelengths, emitting powers, and other properties. More recently, Organic Light Emitting Diodes (OLEDs) have increasingly gained popularity. They are based on an electroluminescent element comprising a series of organic thin films between two conductors.
A key problem related to using LED sources is the fact an LED can not be easily collimated or shaped. This follows from the fact that a LED is an extended light source, i.e. a source where the light originates from an extended region of space (typically an LED's active area is a few tens of micrometers up to a few hundreds of micrometers or up to a few millimeters, whereas, for example, a vertical cavity surface emitting laser's (VCSEL's) active area is few micrometers). Consequently, LEDs typically have an output light distribution that extends over a broad angular range. If there is the requirement that the light produced by the LED be focussed, collimated or otherwise influenced, an external optical system has to be arranged next to the LED.
An example of an LED according to the state of the art comprising such an optical system is shown in FIG. 1. This figure schematically illustrates a set-up comprising an LED consisting of an LED chip 1, a housing 2 which acts as a diffuse reflector redirecting light not emitted into the upward direction, and epoxy 3 (or other transparent material) surrounding the chip. The set-up further comprises an external optics 4 added on top of the epoxy layer by means of an appropriate holder 5. The holder has to be such that there are no poorly defined interfaces between the LED source and the external optics 4. For example, an thin irregular air gap between these components could lead to optical distortions. The holder may for example be an adhesive layer having appropriate optical characteristics. In the conventional case, the optics 4 is usually a so called dome lens, which collects light emitted by the chip and weakly collimates it. A typical output obtained with such setup follows Lambert's cosine law, i.e. intensity distribution is directly related to the cosine of the angle the source is viewed from.
A more narrow angular distribution can only be achieved by means of complicated reflector and/or lens systems. A complicated system of optical components is also necessary in order to additionally shape the emitted light beam.
In the prior art it has been suggested to use diffractive optical elements (DOES) in conjunction with LEDs. WO 97/04491 discloses a DOE as a replacement for a spherical or aspherical lens shaped in a glass carrier on which a LED is mounted. EP 1 115 155 A2 shows a optical computer with an array of LEDs arranged under a glass plate comprising electrodes for contacting the LEDs and an array of DOEs for collimating light emitted by the LEDs.
LEDs are often used as components of LED display panels comprising an array or irregular arrangement of LEDs. The mounting of LEDs comprising a dome lens in such an LED panel requires separate holding means if the dome lens is not to protrude from the top surface of the LED panel, and the mounting of an additional reflector and/or lens system is too complicated for most applications. Further, due to the size of the dome shaped lenses, the LED panels having these state of the art collimation means have a considerable minimum thickness. In other applications, such as interior lighting, light coupling into back-or front light displays, projection displays, LCD projectors or flash lights for miniature cameras, the compactness of the overall light emitting device is of major importance.